Current practice in the field of fluid distribution systems is to rely on direct acting or pilot operated pressure relief valves to avoid excess pressure developing within a system, thereby exposing attached components to damaging overpressures. Relief valves are normally connected in the system so that they discharge fluid to waste or a lower pressure zone when the system pressure exceeds a predetermined set point pressure.
Direct acting relief valves rely on a valve member that is exposed to an opening force created by the pressure of the fluid distribution system and a closing force created by a spring. If the pressure of the distribution system (the opening force) exceeds the closing force established by the spring, the valve will open to release fluid and relieve pressure until the spring force is sufficient to close the valve. Sometimes a dashpot is used to reduce fast closing or slamming of the valve.
Direct acting relief valves offer the advantages of opening quickly and being useable in both clean and dirty fluid distribution systems. At the same time, such valves currently suffer from the disadvantage that once open, the valves often do not close again until the system pressure has decreased significantly—usually well below the established overpressure limit. The lag time between the opening and the closing of the valve sometimes results in significant fluid loss. The lag is primarily due to the presence in typical pressure relief valves of elastomeric seals between the valve body and the valve actuator. The elastomeric seals are normally very tightly fit to prevent leakage, but a tight seal contributes to valve hysteresis. In highly sensitive applications, such as aerospace applications, in addition to fluid loss the relief valve lag may also cause valve chatter, unwanted vibrations, and unacceptable pressure fluctuations.
As an alternative, sometimes pilot operated relief valves are used to relieve overpressures in the fluid distribution system. Pilot operated valves employ a small direct acting valve to control a larger diaphragm or piston operated main relief valve. The pilot operated valves require needle valves, strainers or filters in the pilot circuit. This characteristic makes them suitable for clean applications, but inappropriate for dirty systems handling particulate material that can clog the pilot circuit.
Pilot operated relief valves also suffer from the disadvantage of relatively slow reactions to overpressure conditions. Because pilot operated valves have to release fluid from a control chamber through tubing and the pilot valve itself, several seconds may be required for the valve to open fully. Booster valves may be used to speed opening, but the time delay may still be too long to suit the operating specifications of the fluid distribution system.
Further, most pressure relief valves are difficult or impossible to adjust for changing applications. Some systems have changing overpressure parameters because of the use of a single apparatus for many different processes. Current pressure relief valves make the use of overpressure systems on apparatus with varying overpressure requirements difficult to maintain.
The present invention is directed to eliminating, or at least reducing the effects of, one or more of the problems described above.